1. Field of the Invention
The present invention relates to a field of circuit design, more particularly to multiple-input comparator and power converter circuits.
2. Description of Related Art
In the prior art, a pulse width modulation system usually employs a pulse width modulation (PWM) comparator. FIG. 1 is a schematic diagram showing a comparison principle of the PWM comparator. EAO is an error amplifying (output) signal outputted from an error amplifier. Ramp is a saw-tooth or triangular wave signal. The PWM comparator is provided for comparing an error amplifying signal EAO with the triangular wave signal Ramp to produce a pulse width modulation (output) signal PWMO. When the error amplifying signal EAO is larger than the triangular wave signal Ramp, the PWM signal PWMO is set at high level. When the error amplifying signal EAO is smaller than the triangular wave signal Ramp, the PWM signal PWMO is set at low level. In other words, the signal level of the PWM comparator changes polarity when the error amplifying signal EAO is equal to the triangular wave signal Ramp.
The operation of pulse width modulation is that the PWM system produces the PWM signal PWMO with different duty cycles along with the error amplifying signal EAO. It can be seen from FIG.-1 that the duty cycle of the PWM signal PWMO increases when the error amplifying signal EAO increases, and the duty cycle of the PWM signal PWMO decreases when the error amplifying signal EAO decreases. For various types of power converters, such as DC-DC converters, DC-AC converters or AC-AC converters, a feedback loop circuit is usually employed for adjusting the duty cycle of a power switch. If the triangular wave signal Ramp has a fixed frequency, the modulation of pulse width is equivalent to the modulation of the duty cycle.
The triangular wave signal Ramp usually is generated by an oscillator. A conventional oscillator outputs a saw-tooth wave signal as shown in FIG. 1. A valley voltage of the saw-tooth wave signal is 0 volt. Using a buck DC-DC converter as an example, a duty cycle is required to equal to VO/VIN when a loop circuit is stable, where VO is the output voltage and VIN is the input voltage of the DC-DC converter. When the input voltage VIN is much larger than the output voltage VO−, the required duty cycle for stable loop circuit is very small. If the valley voltage of the saw-tooth wave signal Ramp is equal to 0 volt, the error amplifying signal EAO needs to be near 0 volt when the required duty cycle is very small. Thereby, an output element of the error amplifier may be in the saturation region and the gain of the error amplifier decreased significantly. As a result, the error amplifier does not function normally. Hence, the saw-tooth wave signal Ramp is required to be enhanced by a certain voltage ΔV. FIG. 2 shows two saw-tooth wave signals before and after voltage enhancement. Ramp1 is the saw-tooth wave signal before voltage enhancement, Ramp2 is the saw-tooth wave signal after voltage enhancement, and ΔV is an enhancement voltage.
FIG. 3 is a circuit diagram showing a conventional circuit for enhancing a saw-tooth wave signal Ramp. Referring to FIG. 3, the circuit comprises an operational amplifier OP1, an oscillator, resistors R1 and R2, PMOS transistors MP1, MP2 and MP3, and NMOS transistors MN1, MN2 and MN3. The oscillator produces the unenhanced saw-tooth wave signal Ramp1. An intermediate node 310 between the resistor R2 and the PMOS transistor MP3 is provided as an output terminal for the voltage-enhanced saw-tooth wave signal Ramp2. The saw-tooth wave signal Ramp2 is enhanced by a voltage of V1·(R2/R1) relative to the saw-tooth wave signal Ramp1. However, the circuit is very complicated and is restricted by the responsive speed of the operational amplifier OP1.
For a conventional current mode power converter, the PWM signal is generated either by adding a sampled inductance current to the saw-tooth wave signal Ramp, and comparing the sum with the error amplifying signal EAO from an error amplifier, or by converting the error amplifying signal into a corresponding current signal, and subtracting the sampled inductance current from the corresponding current signal, then converting the difference back into a voltage signal, and comparing the saw-tooth wave signal after voltage enhancement Ramp with the voltage signal.
FIG. 4 is a schematic circuit diagram showing a conventional circuit for generating the PWM signal by subtracting a sampled inductance current ISEN from an EAO of an error amplifier and comparing the difference with the voltage-enhanced saw-tooth wave signal Ramp. Referring to FIG. 4, the circuit comprises an operational amplifier OP2, an oscillator, an enhancement circuit, resistors R3 and R4, PMOS transistors MP11 and MP12, NMOS transistor MN11, a current sampling current source, and a PWM comparator. The resistance values of resistors R3 and R4 may be identical equal. In the circuit shown in FIG. 4, RampSH is the enhanced saw-tooth wave signal Ramp. EAO_ISEN is an output voltage showing the difference between the current sampling signal ISEN and the error amplifying signal EAO. The PWM comparator is provided for comparing the output voltage EAO_ISEN with the enhanced saw-tooth wave signal RampSH to produce the PWM signal PWMO. The circuit is very complicated and is restricted by the responsive speed of the operational amplifier OP2.
Thus, improved techniques for a PWM comparator are desired to overcome the above disadvantages.